Digital subscriber line (DSL) has quickly emerged as a high quality solution for high speed Internet access and other services associated with high speed Internet services, such as, voice over Internet protocol (VoIP) and streaming video services. DSL can transmit both voice and data simultaneously over an existing, single copper pair up to 18,000 feet long. Since DSL can utilize existing copper telephone lines, the service costs associated with DSL is relatively low for service providers and for customers. Moreover, since data can be transmitted relatively quickly using DSL, it is a very attractive option for providing high-speed access to end users.
Traditional plain old telephone service (POTS) uses a narrow 4-kHz baseband frequency to transmit analog voice signals, and current modem technology can achieve a data transmission rate of up to 56 kb/s. DSL, e.g., asymmetric DSL (ADSL), can increase the usable frequency range from 4 kHz to 1.1 MHz and can provide a data transmission rate up to 8 Mb/s. Further, frequency division multiplexing (FDM) can allow ADSL to create multiple frequency bands that can be used to carry data simultaneously with POTS signals over the same copper pair. The lower 4-kHz frequency range is reserved for POTS, the middle frequency band is used to transmit upstream data and the larger, higher frequency band is used to transmit downstream data.
Discrete multi-tone (DMT) modulation is the American National Standards Institute (ANSI) standard T1.413 line code. DMT modulation is used to divide the data bandwidth into 256 subchannels, or tones, that range from 20 kHz to 1.1 MHz for ADSL. Upstream data transfer frequencies range from 20 kHz to 160 kHz and downstream data transfer frequencies range from 240 kHz to 1.1 MHz. The remaining tones are used as guard bands for dividing the three frequency bands, and one pilot tone is used in each data stream, both upstream and downstream, for timing purposes. Each tone, or channel, has a spacing of 4.3 kHz and each tone supports a maximum number of 15 bits, which is limited by the signal-to-noise ratio on the channel. Since the tones in the higher frequencies are subject to higher attenuation and noise, the number of bits per tone can be fewer than that in the lower frequencies.
In addition to the normal data bits, an embedded operations channel (EOC) is provided as part of the ADSL protocol for communication between the ATU-C and the ATU-R to provide in-service and out-of-service maintenance, to retrieve a limited amount of ATU-R status information, and to monitor ADSL performance.
Typically, the optimization of the data transmission channels used for DSL data transport, e.g., VoIP and video, is largely ignored due to technical and economic factors. Without optimization, DSL circuits are either over-engineered or under-engineered for performance. Over engineered circuits operate at sub-optimum rates and deliver less performance to the customer. Under-engineered circuits experience frequent data errors that result in increased peer-to-peer communications required to perform re-transmissions of data packets. This yields a reduced throughput to the customer. Some under-engineered circuits experience error rates severe enough to cause service interruption or the inability to establish the data channel when initially requested by the customer.
Accordingly, there is a need for a system and method for optimizing digital subscriber line data service.